Sometimes if your finger is not squarely on the button when firing, it can close the circuit of the switch to a point where it is barely on and can go on and off while you are hitting it, winding you up into the menu. The -- indicates you were between menu choices. It should have gone blank after about five seconds. If it didn't go off then maybe some component on your board is beginning to fail.
I have a unit that is about 4 years old now, and it is labeled for parts now. It will work fine for a while, then it will cut off and on while firing, other times it will fire fine. Sometimes it won't fire at all, but If I pull the battery for a day or so, it will work for a little while.
I'm keeping it because I will pull it apart some day, and keep the screen and stuff in case I damage another, and the top cap might get used on a home made box mod.